It's pretty simple. The hardest part is converting the old speeds to current values. If you were to use a 100 speed film a direct value would be 100 @ 16.

Make your ISO a fraction and read it like this.......1/160th your f stop is still 16 for bright sun.
ISO 25 would be 1/25th @ 16.
This works for any speed film. You just need to use a speed that's close. On your camera that's 1/100 @ f22 or 1/300 @ f11.
That's ball park or calibrated eyeball(sunny 16)

Since the camera doesn't use the "modern" shutter speeds, use sunny f16 (or whatever is appropriate for the north of England for this time of year and err on the side of more exposure rather than less. So if proper exposure on a modern camera might be 1/125 at f8, make it 1/100 at f8. With negative film, always err on the side of overexposure -- it actually does better that way. So maybe just rate the film at 100 speed and go from there.

“You seek escape from pain. We seek the achievement of happiness. You exist for the sake of avoiding punishment. We exist for the sake of earning rewards. Threats will not make us function; fear is not our incentive. It is not death that we wish to avoid, but life that we wish to live.” - John Galt

I'm not sure that is the case for the shutter speeds - some cameras yes, but does it apply for this Rollieflex's shutter?

And to the OP: less than one stop over-exposure would most likely work very well with Portra 160 and most other negative films. If you decide to shoot slide film, get back in touch!

Matt

“Photography is a complex and fluid medium, and its many factors are not applied in simple sequence. Rather, the process may be likened to the art of the juggler in keeping many balls in the air at one time!”

Ansel Adams, from the introduction to The Negative - The New Ansel Adams Photography Series / Book 2